In the optical disk apparatus, an optical beam spot is formed at a position of the target track on the optical disk to follow the track. In this case, an actuator to move an optical head along track cross direction is driven. This actuator moves the optical head from most inner track to most outer track on the optical disk. Positioning of the optical head is required by accuracy within "1/20" track width because of recording and/or reproducing information.
Recently, in order to realize the positioning of the optical head, a linear actuator for positioning of a large displacement is used. The linear actuator is supported to move along track cross direction by a guide shaft. In case the positioning of high accuracy is required, a coarse actuator for positioning of the large displacement and a fine actuator for positioning of a fine displacement are cooperatively used. However, inspite of cooperative use of both actuators, if the friction between the actuator and the guide shaft is generated, the positioning is not accurately executed.
FIG. 1 is a block diagram of the disk apparatus (tracking control apparatus) according to a prior art. In the tracking method of the prior art, an error between a position of the target track and a position of the optical beam spot is optically detected as a tracking error signal and used as an error signal. In FIG. 1, the optical beam spot is reflected on the optical disk. This reflection light is detected by a two split photo detector 12 and converted to an electric signal. The tracking error signal is amplified and filtered by an adder-subtractor amplifier circuit 13 and converted to a driving signal by a tracking compensation circuit 14. The driving signal is supplied to a coarse actuator driving circuit 15 to drive a coarse actuator 16 and a fine actuator driving circuit 17 to drive a fine actuator 18. These actuators are driven to compensate the tracking error. As a result, the optical beam spot follows the target track. In this case, the tracking compensation circuit 14 executes filtering calculation and gain multiplication. In general, high frequency elements are mainly inputted to the fine actuator driving circuit 17 and low frequency elements are inputted to the coarse actuator driving circuit 15. The coarse actuator 16 is supported by a bearing for the guide shaft. A friction generated between the bearing and the guide shaft disturbs the tracking of the optical head 4 to follow the target track. In order to avoid this problem, a roll bearing, whose friction is small is often used. However, a thickness of this part is large because rolling balls are existed between the guide shaft and the bearing. Furthermore, location of the bearing is limited because support force generated by the bearing is equally distributed.
As another method to solve this problem, a slide bearing is used as the bearing part. By using the slide bearing, a design flexibility is high and an element cost is cheap because the optical head and the bearing are formed as one unit. Furthermore, a size of the actuator driving system becomes thin because a distance between the guide shaft and the bearing is short. However, the friction between the guide shaft and the slide bearing is largely generated and the positioning error is also large because of the friction. Concretely speaking, a movement of the actuator is stopped by the friction and continuously stopped for a predetermined time till a force larger than the friction is supplied. Therefore, the error between the target track and the optical head continuously increases during the predetermined time. If the static friction is large, increase quantity of the error is also large because the predetermined time becomes long.
In case of actual positioning, a tracking servo system is comprised of filtering operation of the tracking compensation circuit 14 in order to suppress a disk eccentricity. If the actuator used for tracking control is affected by a large friction, the disk eccentricity is sufficiently suppressed as shown in FIG. 2. However, if the increase quantity of the error generated by the friction is large, a maximum of a remaining error disturbs the suppression of the disk eccentricity.
On the other hand, several methods to compensate the friction are considered. For example, a speed sensor detects a stop status of the actuator for the friction. Then, the stop status of the actuator is released by supplying a large driving force to the actuator. However, in this method, reliability of output signal from the speed sensor at speed "0" timing is low and the speed sensor can not be attached to the actuator because the cost is high and the construction of the tracking servo system is complicated. Furthermore, a direction of the driving force supplied to release from the stop status of the actuator is not determined if a moving direction of the target track is not detected. In short, the stop status of the actuator is not detected by a simple method and such friction compensation is not actually executed in case of tracking control.